The present invention relates to a wire electric discharge machining apparatus.
A conventional wire electric discharge machining apparatus, as shown in FIG. 3, includes a base 1, a table 2 and a linear guide means 3 for guiding the table 2. The linear guide means 3 is composed of rails 3a fixed to the base 1 and extending in a direction of a Y-axis thereto, and bearings 3b engaged with the rails 3a and mounted on the table 2. The table 2 is moved in the Y-axis direction relative to the base 1 through a feed screw connected to a motor 4 mounted on the base 1. A loading surface 5 of a work mounting base on which a workpiece 6 is loaded is set in parallel with the moving surface of the table 2. An inverted U-shaped column 7 is fixed to the base 1, with the column 7 having a linear guide means 9 for guiding a frame 8. The linear guide means 9 is composed of rails 9a fixed to-column 7 and extending in an X-axis direction perpendicular to the Y-axis relative to the column 7, and bearings 9b engaged with the rails 9a and mounted on the frame 8. A motor 10 is mounted on the column 7 for moving the frame 8 in the X-axis direction relative to the column 7 through a feed screw. The frame 8 is C-shaped and carries a Z-axis quill 11 on its upper arm portion. The frame 8 carries, on its lower arm portion, a lower guide 13 having a nozzle for supplying a machining liquid in a coaxial manner with a wire 12 while guiding the wire 12. A motor 14 is mounted on the frame 8 for moving the Z-axis quill 11 through a feed screw in the Z-axis direction perpendicular to the X- and Y-axes relative to the frame 8. A flange 15 is mounted at a tip end of the Z-axis quill 11. A U-V axis moving unit 16, removable from the flange 15, has a V-axis moving means 17, a linear guide means 18 for guiding the V-axis moving means 17, a motor 19, a U-axis moving means 20, a linear guide means 21 for guiding a U-axis moving means 20 and a motor 22. The linear guide means 18 is composed of rails 18a fixed to the flange 15 and extending in a direction of a V-axis in parallel with the Y-axis relative to the flange 15, and bearings 18b engaged with the rails 18a and mounted on the V-axis moving means 17. A motor 19 is mounted on the flange 15 for moving the V-axis moving means 17 through a feed screw in the V-axis direction relative to the flange 15. A linear guide means 21 for guiding a U-axis moving means 20 is composed of rails 21a fixed to the V-axis moving means 17 and extending in a U-axis direction in parallel with the X-axis relative to the V-axis moving means 17, and bearings 21b engaged with the rails 21a and mounted on the U-axis moving unit 20. A motor 22 is mounted on the V-axis moving means 17 for moving the U-axis moving means 20 in the U-axis direction relative to the V-axis moving means 17. An upper guide 23 mounted on the U-V axis moving unit 16 has a nozzle for supplying a machining liquid in a coaxial manner with the wire 12 while guiding the wire 12 in the same manner as the lower guide 13. Incidentally, the reference position of the upper guide 23 is a vertical position relative to the lower guide 13, i.e., a position where the wire 12 is kept vertically to the loading surface 5. The frame 8 is provided with a reel 24 and a pair of takeup rollers 25. The wire 12 wound around the reel 24 is introduced into the upper guide 23 and is passed through the lower guide 13 to be wound by the rollers 25.
When a workpiece having parallel upper and lower surfaces 6 is loaded on the loading surface 5 and the upper guide 23 is set at the reference position, i.e. the starting point of machining, the wire 12 is held vertically to the upper surface of the work 6. In order to prevent any electric discharge in the air and to remove sludge generated during the machining operation, the machining liquid is supplied from the upper guide 23 and the lower guide 13 in the coaxial manner with the wire 12 and then an electric energy is supplied thereto. When the table 2 and the frame 8 are moved, respectively, the wire 12 and the workpiece 6 are relatively moved in the X- and Y-axis directions. Thus, it is possible to perform a so-called vertical machining in which the machined surface is kept vertically relative to the upper and lower surfaces of the work 6. In the same manner, when the relative position between the upper guide 23 and the lower guide 13 is changed by the U-V axis moving means 16 to impart an inclination to the wire 12, it is possible to perform a so-called inclined machining in which the machined surface is inclined relative to the upper and lower surfaces of the workpiece 6.
In order to enhance a machining precision, it is necessary to always hold the wire 12 in a predetermined position relative to the workpiece 6. Therefore, the column 7 and the frame 8 that are structural members have a structure as subjected to rigidity. Furthermore, the interval between the rails 3a is designed to be large and the linear guide means 3 is subjected to relatively large rigidity.
In general, the thickness of the work 6 is much smaller than the longitudinal and lateral dimensions, i.e., dimensions in the X- and Y-axis directions. Thus, the moving distances of the upper guide 23 in the U- and V-axis directions may be shortened. Also, if the type of machining is taken into consideration, there are many more cases in the vertical machining than in inclined machining. Further, if the V-axis moving means 17 and the U-axis moving means 20 are made small in size and light in weight, then it is possible to miniaturize the motors 19 and 22 and the linear guide means 18 and 21. As a result, it is possible to make the U-V axis moving unit 16 small in size and light in weight. Accordingly, it is economically advantageous that, for instance, a single U-V axis moving means 16 is provided for a plurality of wire electric discharge machines that are not provided with the U-V axis moving means, and when the inclined machining is necessary, the U-V axis moving means 16 is loaded on the wire electric discharge machine.
Recently, it has been necessary to enhance the machining speed as well as the machining precision. In order to increase the machining speed, it is necessary to supply the machining liquid to the parts to be machined, without fail. It is effective that the upper guide 23 and the lower guide 13 are close to each other as much as possible and the pressure of the machining liquid is increased.
However, the upper guide 23 is positioned through the four guide means, i.e., the linear guide means 9, 18 and 21 and the guide means for the Z-axis quill 11 with respect to the fixed column 7. Furthermore, since the linear guide means 18 and 21 are small in size, the rigidity thereof is low. Accordingly, if the high pressure machining liquid is supplied, the position of the upper guide 23 relative to the lower guide 13 is unduly offset or displaced due to the reaction of the machining liquid. Namely, the wire 12 is offset relative to the workpiece 6 to deteriorate the machining precision. If a sufficient rigidity would be applied to the U-V axis moving unit 16 in order to prevent the generation of the positional offset of the upper guide 23, the outer dimension thereof would be unduly increased, thereby the weight thereof. This would make it difficult to operate the unit and also, the guide means for the Z-axis quill 11 would be enlarged.